1. Field of the Invention
The present invention relates to an optical disc and an optical disc drive apparatus.
2. Description of the Related Art
In recent years, optical discs that have advantageous characteristics of small size, large storage capacity, and high speed accessibility have become common at high pace. In each recordable optical disc (including a writable disc and a rewritable disc), grooves that cause record/reproduction laser light to be tracked to record tracks are pre-formed on a signal record surface in a concentric circle shape or a spiral shape. In other words, record tracks are formed along grooves formed in the concentric circle shape or the spiral shape. Data is recorded in grooves and/or areas between grooves (these areas are referred to as lands).
In such an optical disc, from view points of easy data handling and convenient data accessing, each record track is divided into sectors in the track direction. Thus, data is processed sector by sector. At the beginning of each sector, management information such as a physical address is pre-recorded as a pit portion. The record capacity for user data per sector is constant.
As rotating method for optical discs, CAV method, CLV method, and ZCAV method (also referred to as MCAV method) are known. In the CAV (Constant Angular Velocity) method, a disc is rotated at a constant velocity (constant number of rotations per second). The number of tracks per rotation on the inner periphery side is the same as that on the outer periphery side. When the number of rotations is constant, since the linear velocity on the outer periphery side of the disc is higher than that on the inner periphery side thereof, the length of the record area per sector increases in the direction of the outer periphery. As a result, in the CAV method, the data record capacity per disc is small.
In the CLV (Constant Linear Velocity) method, the linear velocity on the inner periphery side of the disc is the same as that on the outer periphery side thereof. In the CLV method, since the linear record density is constant, the data record capacity is large. On the other hand, it is necessary to control the rotation velocity of the disc corresponding to the position in the radius direction. Thus, the access velocity of the disc becomes low. In other words, the CLV method is not suitable for a recordable disc that requires high accessibility.
In the ZCAV (Zoned Constant Angular Velocity) method, using the advantages of easy rotation control and high accessibility of the CAV method, the disadvantage of which the record capacity is low can be solved. In the ZCAV method, the record surface of the optical disc is divided into a plurality of groups in the radius direction of the disc so that the basic frequencies of the groups differ from each other. These groups are referred to as zones. In each zone, one track is divided into several ten sectors to several hundred sectors. In each zone, the number of sectors is constant. Thus, an outer zone has more sectors. In each zone, the length (in the peripheral direction) of the record area per sector is almost the same. In the ZCAV method, the number of rotations of the disc differs in each zone. In each zone, the number of rotations of the disc is constant. The clock frequency differs in each zone.
FIG. 1 shows the structure of an optical disc corresponding to the ZCAV method. In FIG. 1, for simplicity, the disc is divided into three zones 101, 102, and 103 in the radius direction. In each zone, tracks are formed in a concentric circle shape or a spiral shape. As an example, grooves are pre-formed. Data is recorded on a land. Each track is divided into many sectors. At the beginning of each sector, a header portion 104 represented by a dirk area is preformed as a pit portion. A pit portion is formed corresponding to addresses information or the like. A header portion 104 is followed by a data record area 105.
For simplicity, in the zone 101 on the outer periphery side, 16 sectors are formed. In the zone 102 on the inner periphery of the zone 101, 12 zones are formed. In the zone 103 on the innermost periphery side, eight sectors are formed. Reference numeral 106 represents the minimum sector length of each zone. In reality, more zones than those shown in FIG. 1 are formed. The number of sectors that differ between adjacent zones is one to several sectors.
In the ZCAV method, the header portions 104 are radially adjacent. In other words, in different zones, the header portions 104 are not aligned. However, normally, in different zones, there is at least one position of which the header portions 104 are radially adjacent. In the example shown in FIG. 1, the header portions 104 are radially aligned at intervals of 90°.
As in FIG. 1, FIG. 2A shows the structure of a conventional optical disc. FIG. 2B is an enlarged view showing a portion 201 in which header portions are radially adjacent in different zones. In FIG. 2B, reference numeral 202 represents a part of a zone on the outer periphery side. Reference numeral 203 represents a part of a zone on the inner periphery side. Reference numeral 204 represents a header area. Reference numeral 205 represents a data record area. In the header area 204, a pit portion 206 is formed. On each track, a pit portion 206 and data are recorded on a land surrounded by grooves 207. A zone boundary is represented by a broken line 208.
As in FIG. 1, FIG. 3A shows the structure of a conventional optical disc. FIG. 3B is an enlarged view showing a portion 301 of which header portions are not adjacent in different zones and of which a header portion and a data record area are adjacent. In FIG. 3B, reference numeral 302 represents a part of a zone on the outer periphery side. Reference numeral 303 represents a part of a zone on the inner periphery side. Reference numeral 304a represents a header area of the zone on the outer periphery side. Reference numeral 304b represents a header area of the zone on the inner periphery side. Reference numeral 305a represents a data record area of the zone on the outer periphery side. Reference numeral 305b and 305c represent data record areas of the zone on the inner periphery side. Pit portions 306 are formed in the header areas 304a and 304b. A pit portion 306 and data are recorded on a land surrounded by grooves 307 of each track. A zone boundary 308 is represented by a dotted line 308. The header area 304a and the data record area 305b are adjacent with the zone boundary 308. In addition, the header area 304b and the data record area 305a are adjacent with the zone boundary 308.
When a header portion and a data record area are adjacent with a zone boundary as shown in FIG. 3B, a crosstalk that takes place from the header portion (pit portion) to the data record area increases. As a result, recorded data may not be correctly reproduced. This is because when a base material of the disc is formed, an optical distortion-that takes place around the pit portion becomes an optical aberration in the data record area. To solve such a problem, the following related art references are known.
In one related art reference disclosed as Japanese Patent Laid Open Publication No. 4-301263, a zone boundary is a buffer area (non-recordable area). However, in the related art reference, there is a portion of which the pit portion of the non-recordable area is adjacent to the data record area. Thus, it is difficult to prevent a crosstalk from taking place in the data record area.
In another related art reference disclosed as Japanese Patent No. 2972900, a zone boundary is formed using only grooves without a pit portion. In the related art reference, the buffer area is effective against a crosstalk. However, when a seeking operation is performed, if this area is accessed, since there is no pit portion that represents an address, position information cannot be obtained.